In every molecular structure, there exists a number of atoms bonded together by electrons that surround the nuclei of the atoms. Each molecule has discrete electronic energy levels and each such level has a characteristic spin value. The theory of quantum mechanics has shown that these energies do not vary continuously. Instead, the energy of an electron can only change in indivisible quantum steps. Each electron surrounding a nucleus is said to exist at one of several possible energy levels that are an integer number of quanta apart in energy. The energy of an electron will not increase to a higher level until the electron absorbs energy equal to the difference between the electron's current energy level and the higher energy level. Knowledge of the energy levels of electrons and the differences between the electron energy levels reveal a great deal of information regarding the structure of a molecule, which can help a scientist better understand the physical position of atoms within the molecule, the strength of the chemical bonds between the atoms, etc.
One of the most useful methods for determining the energy levels of electrons is called electron spin resonance (ESR). Using this method, a single crystal of a material being studied is placed in a strong magnetic field. The sample is then exposed to a high-frequency electromagnetic signal. A detector is used to measure the extent to which the electromagnetic signal is absorbed by the sample. The absorption versus the strength of the magnetic field is plotted to produce an ESR spectrum of the molecules in the material.
Despite the success of the ESR technique, it suffers from several drawbacks that limit its usefulness in detecting spin transitions for all substances. First, in order to obtain the maximum geometric information from the ESR technique, the sample under investigation must be in the form of a single crystal. For many substances, it is difficult, if not impossible, to obtain single crystals. For example, many biological materials cannot be crystallized. Secondly, in a standard ESR experiment, the absorption frequencies for the magnetic resonance transitions change depending on the crystal's orientation in the magnetic field. For chemical substances such as powders, the molecular fragments are randomly oriented within the magnetic field, which makes interpretation of the spectral information difficult or impossible. Therefore, there is a need for a diagnostic tool or technique that allows a researcher to determine the magnetic resonance parameters of a paramagnetic substance accurately without requiring that the sample be available in crystalline form.